As is well known, a refrigerator performs a cooling or freezing function through a cooling system thereof. Usually, the cooling system includes three essential elements, namely, an evaporator, a compressor and a condenser. In operation, the compressor compresses a low temperature and low pressure refrigerant gas from the evaporator into a high temperature and high pressure refrigerant gas and the condenser condenses the high temperature and high pressure refrigerant gas from the compressor to a high temperature and high pressure refrigerant liquid. The high temperature and high pressure refrigerant liquid is changed into a low temperature and low pressure refrigerant liquid through an expansion valve, and then flows into the evaporator. The low temperature and low pressure refrigerant liquid absorbs heat from an air around the evaporator, thereby cooling the air. In order to efficiently cool the air to thereby increase the cooling efficiency, one or two fans are usually employed in the refrigerator.
In the refrigerator employing one fan, the operating rates of the fan and the compressor are determined on the basis of the interior temperature of the refrigerator compartment, whereas in the refrigerator employing two fans, those of each of the fans and the compressor are determined by the interior temperature of the refrigerator compartment and the freezer compartment, respectively.
Such refrigerators, however, have a shortcoming in that there is no difference between nighttime and daytime in the level of operating noise produced by the fans and the compressor which constitute the principle sources of refrigerator operating noise, since the refrigerator is incapable of distinguishing whether or not it is nighttime or daytime.